Infrared proximity detectors are used to detect the presence of an object in the vicinity of machinery or automatic doors so as to inhibit operation of the machinery, or to open and close automatic doors. Infrared radiation is used because it provides an invisible beam in a sensing region adjacent the machinery or automatic doors. GB-A-2 207 999 discloses such a proximity detector in which an array of infrared light emitting diodes illuminate a reflector strip mounted in the object plan of a camera having a linear array of infrared sensors to detect the radiation returned by the reflecting strip. This arrangement provides a "curtain" of sensitivity over a wide area When an object passes through the "curtain" a dark shadow is obtained on the photosensitive array within the camera due to the appreciable difference in reflectivity between the reflector strip and the object. Therefore, the reflector strip is an important feature of this arrangement. Moreover, the circuitry used to process a proximity signal is adapted to respond to the shadow on the photosensitive array.
GB-A-2 166 831 also discloses an arrangement in which a curtain of radiation is spread across a detection zone and is returned, by a prismatic reflector, back to a photodiode array. However, in this case, a complicated optical arrangement is used to produce the curtain of radiation from an infrared source and to return the reflected radiation. The reflector is again an important feature of such an arrangement.
Besides the foregoing arrangements, which employ reflectors, proximity detectors have also been proposed which depend on using a receiver for radiation reflected directly from an object which intercepts beams of infrared radiation or acoustic energy. An example of such a system is disclosed in EP-A-226322. However, in such systems, the radiated energy is in the form of a cone with a maximum intensity on the conical axis and the reflected component is small. Therefore, complicated arrangements of e.g. a plurality of infrared transmitters and receivers need to be used to ensure that sufficient energy is radiated into the detection zone to detect objects of various sizes and/or having different reflectivities and to ensure that a detectable reflected component is returned from the object to the receiver to act as a trigger for proximity detection. These infrared transmitters and receivers are also mounted in spaced locations on a doorframe thereby entailing the use of complex wiring and expensive installation.
It would therefore be advantageous to provide a system which detects, in ambient light conditions, the presence of an object intercepting an infrared beam of radiation, which does not require the use of a reflecting strip, and which avoids the need for a plurality of transmitters/receivers as well as the need to install complex wiring. The present invention seeks to solve this problem by providing a laser light transmitter which can be used in an improved proximity detector system.